The present invention relates to antenna pointing and tracking systems and, more particularly, to antenna pointing and tracking systems where a station at each end of a two-way radio link is required to track a signal emitted by the station at the other end.
Directional antennas are employed for maximizing the effective radiated power in the desired direction and for minimizing the reception of noise originating in undesired directions. In a two-way communications, link, with each end of the link employing a directional antenna, maximum performance is achieved when the peak of each antenna pattern is centered on the antenna at the other end of the link. In terrestrial applications, where both ends of the link are stationary, pointing of the antennas does not present a difficult problem.
In space applications, the bilateral pointing problem becomes significant. In an earth-satellite-earth communications system, or a satellite-to-satellite communications system, the angular position of the station at each end of the link continuously changes with respect to the station at the other end of the link. Thus, it is desirable to dynamically aim the antennas for centering the beam axis of each on the location of the other.
The principle of reciprocity requires that the radiation patterns for an antenna at a given frequency must be identical for both receiving and transmitting.
In a communications link where transmission is continuous at both ends, and tracking is performed using the power transmitted at the other end, it has been found that conventional conical-scan-type tracking cannot be performed at both ends simultaneously. An attempt to track at both ends is upset by the fact that the received signal at each end is modulated not only by the scanning of its receive beam pattern but also modulated by the scanning of the transmit beam pattern at the other end of the link. This results in hunting at both ends of the link and produces unsatisfactory tracking accuracy.
One skilled in the art would recognize that the principle of reciprocity does not hold precisely true when different transmitting and receiving frequencies are employed in an antenna. However, for the present disclosure, insignificant error will result from assuming identical transmit and receive beam shapes.
In order to solve the bilateral conical scanning problem, prior systems have employed monopulse tracking in which four receive beams, slightly offset from the antenna axis, are processed by separate receivers to yield error signals for tracking. A monopulse system simultaneously processes a single received signal and the hunting problems encountered in conical scanning are avoided. Monopulse systems require large, heavy and expensive antennas and quadruplication of receive channels.
Conical scanning systems per se have long been employed to aim directional antennas. Typically, a conical scanning device employs some means for rotating or nutating a transmitted or received beam in a pattern about the axis of the antenna. If the antenna employs a reflecting dish and a radiator, the pattern can be generated by offsetting the radiator from the reflector axis and rotating either the radiator or the reflector to describe a figure, usually a conical figure, about the axis. Such tracking devices relate the angular position of the beam to the magnitude of the signal level received. The relationship between the received signal and the angular position provides the information for aiming the axis of the antenna.
One type of conical scanning employed in a radar system is disclosed in U.S. Pat. No. 2,480,171.
It is not necessary to process the received signal from the entire conical scan. For example, it may be satisfactory to select samples at specific angular positions for determining the target angle. For example, U.S. Pat. No. 2,537,952 discloses a radar system which produces four transmitted pulses per revolution of the conical scanning device so that it receives one target sample per quadrant. The received signals from opposite quadrants are displayed together to guide antenna steering.
The general principle of the above-cited prior art, although applied to monostatic radar tracking systems, is generally applicable to tracking in a communications link. The problem remains, however, of permitting each end of the link to track the other end with a simpler scanning system than a monopulse system.